Esterification processes that manufacture plasticizers are typically run in the liquid phase using homogeneous catalysts. These catalysts must be removed from the plasticizer effluent and this entails extra processing steps as well as creating waste water. A highly crystalline heterogeneous catalyst on the other hand, may be easily recovered by filtration or any other suitable means and recycled to the next reaction batch. This has advantages in that extra processing steps are eliminated, waste water is not generated, and spent catalyst disposal is minimized.
Amorphous titanium containing heterogeneous liquid phase catalysts are known in the art, but results are typically poor. The difference between a zeolite and the amorphous materials is that the Ti is incorporated into the lattice framework of the crystal; it is an inherent part of the molecular structure. In contrast, the amorphous catalyst consists mainly of TiO.sub.2 adsorbed or coated on the surface of a carrier. The Ti is not an integral part of the carrier structure.
U.S. Pat. No. 4,032,550 discloses the preparation and use of titanium supported on monmorillonite clay as well as alumina and silica as catalysts for esterification and transesterification reactions. The method of preparation is complicated and typically Ti concentrations as high as 35% are disclosed. The Ti level is much higher than in the Ti zeolites.
U.S. Pat. No. 5,183,930 discloses the preparation of amorphous TiO.sub.2 /SiO.sub.2 and TiO.sub.2 /Al.sub.2 O.sub.3 catalysts for transesterification reactions. Amorphous Ti(OH).sub.x is formed via the controlled hydrolysis of a Ti alkoxide, and is then put on the carrier. However, this material is not used as a direct esterification catalyst.
U.S. Pat. No. 3,056,817 discloses the use of hydrous titania gel. The gel is distinguished from dry titania, and is said to have no activity as an esterification catalyst. The titania gel is not an effective catalyst at reaction temperatures above 150.degree. C.
U.S. Pat. No. 2,928,853 discloses direct esterification catalyzed by TiSO.sub.4 on a carrier such as a clay. Again, this is an amorphous material and suffers from the disadvantage of sulfur contamination.